In game, Nebula (by-default) is viewed as the lightest blue regions of space. These regions spawn celestial bodies with three different biome types - Forest, Grassland, & Ocean. The notable differences to other space-types is that Nebula doesn’t spawn celestial bodies higher than level 4, the planets which spawn can only be upgraded a maximum of 3 times, and Spacetime Rips & Foundries don’t spawn within these regions (unless playing on custom maps where these parameters are removed).
The colouration of the background where Nebula is the Space-Type (fancy-shaders on)
A peek at the variety/design of biomes which can spawn within Nebula - Forest, Grassland & Ocean!
The Nebula space-type will most often be where a players spawns into the universe (generally, parameters are set so that you must spawn within Nebula), which will allow them to begin their growth safely. This safety is encouraged for two reasons: celestial bodies which spawn in Nebula have very high defense, making them very costly to try to take control from another player; and they have reduced maximum level, upgrades & other stats, making them less effective planets even if another player was to take control of them.
As you continue to expand into deeper and deeper space, you will begin to find that Nebula tend to become zones which hinder your expansion in particular directions due to the nature of the planets and other celestial bodies which spawn (or don’t spawn) within them. On a macro gameplay level, they can represent effective barriers between large empires, which therefore offers some protection/stress-relief.